Slip casting aqueous slurries of high melting point pitch and carbonizing to form carbon articles

ABSTRACT

AQUEOUS BALL MILLED SLURRIES HAVING SOLIDS CONTENTS OF 5-50% BY WEIGHT CONTAINING WATER, PITCH HAVING A SOFTENING POINT ABOVE 170*C., AND 50 TO 80% BY WEIGHT ETHYL ALCOHOL, BASED ON THE WEIGHT OF PITCH, ARE SLIP CAST IN POROUS GYPSUM PLASTER MOLDS OF THE DESIRED SHAPE. THE CAST ARTICLES ARE THEN DRIED AND CARBONIZED. WHEN THE SOFTENING POINT OF THE PITCH IS BELOW 300*C. THE METHOD INCLUDES A FURTHER STEP OF HEATING THE DRIED ARTICLE IN AN OXIDIZING ATMOSPHERE TO 260-300*C. AT A HEATING RATE OF 0.2-3*C./MIN. PRIOR TO THE CARBONIZATION STEP. A FURTHER STEP OF GRAPHITISING IS ALSO DISCLOSED.

United States Patent Int. Cl. c0111 33/28, 35/52 US. Cl. 26429 4 Claims ABSTRACT OF THE DISCLOSURE Aqueous ball milled slurries having solids contents of 550% by weight containing water, pitch having a softening point above 170 C., and 50 to 80% by weight ethyl alcohol, based on the weight of pitch, are slip cast in porous gypsum plaster molds of the desired shape. The cast articles are then dried and carbonized.

When the softening point of the pitch is below 300 C. the method includes a further step of heating the dried article in an oxidizing atmosphere to 260300 C. at a heating rate of 0.2-3 C./ min. prior to the carbonization step. A further step of graphitising is also disclosed.

BACKGROUND OF THE INVENTION This invention relates to the production of formed articles of carbon and more particularly to a new method for producing formed carbon articles by magmatic casting (similar to slurry casting or slip casting in ceramics and hereinafter referred to as slurry casting).

In general, moulding and extruding are practiced as methods for forming formed carbon articles, suitable blends of coke as an aggregate and pitches as binders or bonding agents being used as starting materials. Examples of products thus produced are electrodes, refractory materials, electrical brushes, and various carbon tubes and plpes.

For special formed articles, somewhat different methods are practiced, but a method for producing formed carbon articles by slurry casting in which gypsum plaster (plaster of Paris) moulds are used has never been considered as far as I am aware. This slurry casting method is a substantially general method in the forming of ceramic articles and is considered to be suitable for forming thin articles of complicated shapes.

The principal reasons why the casting forming technique has not been adopted up to now for forming carbon articles are probably as follows. Most of these carbon articles are generally made from coke as an aggregate and pitches as binders, and rendering of mixtures of these materials into the form of slurry for casting has been considered to be ditficult. It has been further considered that, even if forming can be thus accomplished, the baking of thin formed articles would be extremely difficult because they contain pitches of low softening points (temperatures).

As a result of studying methods of producing carbon materials with modified pitches of high softening points, and not combinations of coke and pitches of low softening points, as the principal starting material, I have found that this slurry-casting forming technique can be applied with high effectiveness also to carbon materials and articles.

SUMMARY OF THE. INVENTION It is an object of the present invention to utilise the above set forth finding and other findings as described 3,565,980? Patented Feb. 23, 1971 hereinafter to provide a method for producing formed articles of carbon by this slurry casting technique, which was heretofore considered to be inapplicable to forming of carbon materials.

Another object of the invention is to provide a method for producing formed articles of carbon in which the porosity, density, hardness, and other properties of the formed articles after baking can be readily controlled as desired.

Still another object of the invention is to provide a method as stated above which is applicable to the production of a wide range of formed articles, including articles of thin thicknesses as small as 1.0 mm.

According to the present invention, briefly summarised, there is provided a method of the above stated character which is characterised by the steps of preparing a slurry containing as its principal carbon material a modified pitch of a softening point above degrees C. prepared by treatment of coal tar pitches, petroleum asphalts, and other pitch-like substances resulting as industrial byproducts, casting this slurry into a formed article, drying the article, then heating the same in an oxidising atmosphere if necessary, then carbonising the article in an inert gas, and graphitising the same if necessary.

The nature, principle, details, and utility of the invention will be more clearly apparent from the following detailed description beginning with certain definitions and general considerations and concluding with specific examples of preferred practical embodiment of the invention.

DETAILED DESCRIPTION The generic term modified pitches is herein used to include collectively pitches of softening temperatures above 170 degrees C., preferably above 200 degrees C. These modified pitches are prepared by subjecting coal tar pitches, petroleum asphalts, and other pitch-like substances resulting as industrial byproducts to heat treatment, extraction by solvent, and treatment with various substances such as ozone, oxygen, oxides of nitrogen halogens, sulphur, and sulphur compounds. In some cases, pitches having functional groups of high thermal reactivity such as halogens and/or oxygen-containing groups are included in the modified pitches as referred to hereinabove.

The temperature herein referred to as the softening temperature (or softening point) is that at which sample powder particles of a diametric size of the order of 0.1 mm. within a sealed tube in a micro melting point apparatus melt and assume a spherical shape.

Through the use of the above designated modified pitches, stable slurries can be very easily made by ball milling or some other process. One reason for this is that the specific gravities of these pitches are lower than those of ceramic starting materials. It has further been found out that, in case the modified pitches used as the starting material possess their softening temperature of below 300 degrees C. or so, these pitches, when they are subjected to baking process after being formed into a desired shape can be rendered by heating in air to a temperature of from 260 to 300 degrees C. at a temperature rise rate of the order of from 0.2 to 3 degrees C./min. into a state, wherein the pitches can fully with stand carbonisation and graphitisation treatment in an inert gas phase as a temperature higher than the above mentioned temperature without exhibiting any definite melting or deformation.

Still further, it has been found out that no heat treatment of this sort as abovementioned is required, if the material pitches possess their softening temperature of above 300 degrees C., or particularly, if they have thermal reactive functional groups. In such cases, the heat treatment in air is not always necessary, and the pitches can be carbonised and graphitised at higher temperature by slow heating in an inert gas directly after forming and drying.

The above mentioned modified pitches can be readily prepared to have different softening points by various treatment processes. In accordance with present invention, the preferable processes are dry distillation, oxidation, and chlorine treatment. For example, by subjecting pitches to dry distillation at a temperature of from 350 to 500 degrees C. as nitrogen gas or air is blown thereagainst, it is possible to produce modified pitches having softening points ranging from 150 degrees C. to temperatures higher than 360 degrees C. And some pitches having such high softening point exhibit ample sintering property in spite of the absence of definite melting points.

Furthermore, by carrying out oxidation treatment by ozone, oxygen in a quantity of the order of 25 percent can be easily introduced into a pitch to improve the thermal reactivity thereof. By slowly heating a pitch in powder form in an atmosphere of chlorine and thus treating the pitch at temperatures up to 250 degrees C. to cause it to contain chlorine, a modified pitch having a softening point above 150 degrees C. and no clearly discernible melting point can be readily produced.

The value of the softening point of a modified pitch for slurry preparation has a great influence on the porosity and hardness of the formed article after baking. According to results I have so far obtained, a minimum value of 8 percent and a maximum value of 65 percent can be obtained for porosity by water immersion, and there is a possibility of extending even further the limits of this range.

While a slurry suitable for the method of the invention may be composed, of course, of only water and a modified pitch, the addition thereto of small quantities of additives such as an alcohol for improving the wetness of the modified pitch during ball milling. Furthermore, the addition also of additives such as emulsions of phenolic and other thermosetting resins, water glass, and phosphoric acid or salts thereof is highly effective in increasing the strength of the product after baking.

In addition, mixtures such as mixtures of modified pitches which do not exhibit softening points and modified pitches of low softening points and mixtures of modified pitches of low softening points and carbon in a fine powder form as, for example, colloidal graphite, carbon black, and raw coke powder can also be used.

In general, modified pitches and other carbon materials can be ground most readily by wet grinding, and a fully stable slurry can be produced in a time of the order of three hours by ball milling. I have carried out long-period grinding and mixing up to 50 hours without any particularly deleterious effect on the characteristics of the products.

While the slurry concentration is an important factor determining the rate of success in casting and thickness of the objective formed articles, I have found that a practical concentration by weight is in the range of from to 50 percent and that a concentration particularly of from to percent facilities the practice of the method of the invention.

The shapes of the carbon articles which can be formed by this method range widely from simple shapes such as sheets and plates, bars, tubes, and crucibles to complicated shapes such as those vessels made in split moulds and asymmetrical vessels. Thin articles of thickness of the order of 1.0 mm. can be readily produced.

In order to indicate still more fully the nature and utility of the invention, the following examples of preferred embodiment thereof are set forth, it being understood that these examples are presented as illustrative only and that they are not intended to limit the scope of the invention.

Example 1 500 grammes (g) of a coal-tar pitch of a softening point (hereinafter abbreviated S.P.) of degrees C. is subjected to dry distillation for 60 minutes at 400 degrees C. as air is blown thereagainst. The resulting residue is heated in powder form in air to 200 degrees C. to prepare a modified pitch of an S.P. of from 198 to 210 degrees C.

25 g. of the modified pitch thus prepared is ground for 8 hours in a ball mill together with g. of water and 20 g. of ethyl alcohol thereby to prepare a slurry.

In an actual instance of practice, a slurry prepared in the above described manner was cast with a pouring time of 5 seconds by means of a separately prepared gypsum plaster mould into a small crucible with a wall thickness of 3 mm. After thorough drying at room temperature and then at 100 degrees C., the crucible was heated in air to 300 degrees C. at a temperature rise rate of 0.2 degree C./ min. Thereafter, the crucible was heated at a rate of 5 degrees C./min. to 1,400 degrees C. in an atmosphere of nitrogen gas to carbonise the same.

The resulting diametric shrinkage after the carbonisation process was 22 percent, and a formed article in the shape of a crucible of a porosity of 9.2 percent and a bulk specific gravity of 1.50 was obtained.

Example 2 A pitch produced industrially as a byproduct of thermal decomposition of naphtha was subjected to dry distillation at 400 degrees C. in nitrogen gas. The resulting residue of dry distillation was ground and then heated to 200 degrees C. in chlorine gas, whereupon a modified pitch containing approximately 10 percent of chlorine and having a softening point of 250 degrees C. was obtained.

10 g. of the modified pitch thus prepared, 100 g. of water, and 5 g. of ethyl alcohol were ground for 5 hours in a ball mill to prepare a slurry.

The slurry thus prepared was cast into a small evaporating dish of a thickness of approximately 1.2 mm. with a pouring time of 3 seconds by means of a gypsum plaster mould. The evaporating dish was baked to a temperature of 1,400 degrees C. under conditions similar to those of Example 1.

As a result, with a diametric shrinkage of 15 percent, a carbon evaporating dish of a thickness of approximately 1.0 mm., a porosity of 25 percent, and a bulk specific gravity 1.29 was obtained.

Example 3 500 g. of a coal-tar pitch (S.P. 85 degrees C.) was heated to 460 degrees C. at a rate of 3 degrees C./min. as nitrogen gas was blown thereagainst. The resulting residue was ground and then subjected to ozone oxidation at room temperature for 10 hours and thereafter to air oxidation up to a temperature of 200 degrees C., whereupon a modified pitch containing 20 percent of oxygen and exhibiting no softening point up to 360 degrees C. was obtained. This modified pitch was used to prepare a slurry in the same manner as set forth in Example 1.

The slurry thus prepared was cast into a pot of a height of approximately 6 cm. and a thickness of approximately 3 mm. by means of a gypsum plaster mould with a pouring time of 5 seconds. After thorough drying, the cast pot was heated to 220 degrees C. at a rate of 2 degrees C./ min. in nitrogen gas and then to 1,400 degrees C. at a rate of 5 degrees C./min. thereby to carbonise the same.

As a result, with a height shrinkage of 10 percent, a formed carbon article of pot shape and a porosity of 55 percent was obtained.

I claim:

1. A method of producing formed articles of carbon which comprises: preparing a slurry having 5 to 50 per cent by weight solids content by ball milling an admixture containing as the principal carbon starting material thereof a pitch having a softening point higher than 170 degrees C., water, and 50 to 80 percent by weight, based on the weight of pitch in the slurry, of ethyl alcohol; forming said slurry into an article by slurry casting in a porous gypsum plaster mold; drying said article thus formed; and carbonizing the article in an inert gas phase; said method being further characterized, when the softening point of the pitch is lower than 300 degrees C., by the additional step of heating the article in an oxidizing atmosphere to a temperature of from 260 to 300 degrees C. at a temperature rise rate of the order of from 0.2 to 3 degrees C./ min. prior to the carbonization step.

2. A method for producing formed articles of carbon as claimed in claim 1 in which said pitch is prepared by dry distillation of coal tar pitches, petroleum asphalts, and other pitch-like substances resulting as industrial byproducts.

3. A method for producing formed articles of carbon as claimed in claim 1, in which said pitch is prepared by oxidation of coal tar pitches, petroleum asphalts, and 25 other pitch-like substances resulting as industrial byproducts with one oxidizing agent selected from the group consisting of ozone, oxygen, air, and oxides of nitrogen,

4. A method for producing formed articles of carbon as claimed in claim 1 in which said pitch is prepared by chlorine treatment of coal tar pitches, petroleum asphalts, and other pitchlike substances resulting as industrial byproducts.

References Cited UNITED STATES PATENTS 2,683,107 7/1954 Juel 264-29 3,077,439 2/1963 Shea, Jr. et a1. 1; 26429 3,258,363 6/1966 Lieb 264--29 3,392,216 7/1968 Otani 26429 OTHER REFERENCES 20 of Ceramic Bulletin at pages 334339.

JULIUS FROME, Primary Examiner J. H. MILLER, Assistant Examiner US. Cl. X.R. 23209.2; 26486 

